Hydrogeology of Deke Island, Pingelap Atoll, Eastern Caroline Islands

An extensive hydrogeological investigation was conducted on Deke Island located on Pingelap Atoll, Ponape State. Field work included installation of observation wells, water level monitoring, installation of a tidal gage, core drilling, surface geological and topographical mapping, geophysical surveys, and water quality analysis. Results from the work performed on the study island indicate a number of previously unknown aspects of atoll island hydrology. The most significant of these aspects is the partial confinement of the freshwater lens system beneath the landward extension of the reef flat plate. Where the plate is absent, water table conditions are prevalent. The plate acts as a leaky confining layer which allows some rainwater to enter the flow system by downward movement under gravity. Approximately 2/3 of Deke overlies the reef flat plate; the remainder overlies sediments deposited behind the reef flat depositional environment. In addition to the documentation of hydraulic conditions within the lens system, the behavior of the groundwater resource was studied and a method of reducing observed water levels in wells to actual sea level was developed. Also, several subsurface hydrogeologic units were identified and their roles within the overall hydrology of Deke were defined. In general, the size and extent of the lens was mapped. Based on the interpretation of study results, an atoll island hydrogeologic (conceptual) model is presented. Finally, some practical implications of the study findings are discussed and recommendations for further work are made